Electric fuse device



A118- 26, 1969 w. F. ElsELE ET AL, 3,464,046

ELECTRIC FUSE DEVI Cf] Filed July 5, 1966 4 2s 2o EX /2 a 24 2! C 27 ze United States Patent O 3,464,046 ELECTRIC FUSE DEVICE n Werner Frank Eisele, Stittsville, Ontario, and Helmut H. Lukas, Carleton Place, Ontario, Canada, assignors to Northern Electric Company Limited, Montreal, Quebec, Canada Filed July 5, 1966, Ser. No. 562,669

. Int. Cl. H01h 85/36 U.S. Cl. 337-238 9 Claims ABSTRACT F THE DISCLOSURE A tensioned fuse wire is stretched across a convex surface along a narrow channel and is thus drawn into close thermal contact with this surface to provide al heat sink effect. The wire will rupture at a central span which is sink eifect.

The device includes contacts for closing an alarm circuit on rupture of the wire; a visual indication on rupture of the Wire; and a convenient method of testing the alarm circuit during normal operation.

This invention relatesvto an electric fuse device, and more specifically to 1an electric fuse device of the type in which provision is made for maintaining the fuse wire under tension. Such devices are known, the reason for maintaining the fuse wire under tension being to enable detection of rupture of the wire by the loss of tension and by the consequent movement of one or more members secured to one or both wire ends. Such a movement can be employed to provide an indication, either mechanical or electrical, of the fact that the fuse wire has been ruptured.

Banks of fuse devices of this type are commonly employed in conjunction with expensive electronic equipment, to provide the latter with protection against electrical overloads, and to alford to an attendant an immediate indication when a rupture occurs.

In order for such a device to be effective and reliable in service, it requires certain characteristics over and above the ability to provide an external indication of when the fuse wire has been ruptured. The most important of such additional characteristics is the ability reliably and safely to interrupt a relatively large short-circuit current (for example` 1000 amps, in a 300 volt circuit).

From the viewpoint of being able to `deal eifectively with a high shortcircuit current, the main problem is to avoid the explosion effect. When a highvoltage-high current is passed through a relatively small wire, the wire tends to explode rather violently. The violence of the explosion is related not only to the size of the Wire and to the melting point of Wire material, but is also directly related to the medium surrounding the wire. Generally speaking, the less free air space that surrounds the wire, the less severe will be the explosion. A common way of taking advantage of this fact is to pass the fuse wire along a cylindrical tube made, for example, of a strong bre material which comparatively tightly surrounds the wire. This material minimizes the free air space and contains the explosion within the tube. An alternate method is to surround the fuse element with sand or similar material. Both these methods of fabrication are, however, relatively expensive, and there are ditiiculties in threading the tubes and fastening the fine wire reliably to the end caps. In addition, the tube itself `adds bulk and consequent size to the overall device. In this latter connection, it should be added that another desirable characteristic of such devices is smallness. Since large numbers of these devices are ommonly arranged in banks protecting electronic equipment, a substantial space saving can be achieved yout of contact with the surface and thus lacks the heat y ICC from minimization of the dimensions of the fuse device.

Another problem that has been encountered in fuse devices of the type in which the fuse wire is constantly maintained under tension is a tendency for the fuse Wire gradually to elongate while in service. The constant heating and cooling of the wire eventually enables it to take on a permanent elongation. Such an elongation may even be enough to cause a false alarm, indicating that the fuse wire has been ruptured when it has not.

It is the object of the present invention to provide an electric fuse device that affords improved performance characteristics, and more particularly performance characteristics that are improved in relation to one or more of the diiiiculties discussed above.

One form of fuse device constructed in accordance with the present invention is illustrated diagrammatically and by way of example in the accompanying drawings. 1t is to be understood that the broad scope of the invention is not restricted to specific details of the device illustrated, such scope being defined in the appended claims.

In the drawings:

FIGURE l is a perspective view of a fuse device constructed in accordance with the present invention;

FIGURE 2 is a cut-away side view ofthe device of FIGURE l;

FIGURE 3 is a view similar to FIGURE 2, but showing the positions of the parts after rupture of the fuse wire;

FIGURE 4 is a section on the line lV-IV in FIGURE 2; and

FIGURE 5 is a section on the line V-V in FIGURE 4.

The device comprises an outer casing 10 which will normally be mounted iixedly in the equipment with which the fuse device is associated. The fuse wire itself is mountedon a sliding mount 11 that is slid into the casing 10, in the manner shown in the drawings. The mount 11 and the parts thereon form an expendable unit, a new such unit being placed in the casing 10 after each rupture.

The fuse mount 11 comprises a flat tray 12 of insulating material having raised portions 13, 14, 15 and 16 projecting therefrom. These raised portions serve to retain in place a fuse holder 17 and a slider 18.

Along its underside the fuse holder 17 defines a confming fuse supporting channel 19 which is U-shaped in cross-section (FIGURE 5) and convex in length (FIG- URE 4) except for a centrally located enlargement or cavity 20 where the channel is widened both in width and depth so as to leave a central span 21 of a fuse wire 22 out of direct physical contact with the insulating material of the fuse holder 17.

The ends of the fuse wire 22 are soldered or welded respectively to ledges 23, 24 which have been pressed out of spring `arms 25, 26. As seen in FIGURES 2 and 3, ends 27, 28 of the arms 25 and 26 are held in place between the tray portion 14 and the fuse holder 17 and make electrically conducting engagement with contacts 29, 30 mounted on the xed casing 10 and having external terminals 31, 32.

The circuit to be protected is applied across terminals 31 and 32 which are thus joined together by the fuse wire 22. On the occurrence of an excess current, the wire 22 is ruptured, which opens the circuit and permits the arms 25 and 26 to spring outwardly, as shown in FIG- URE 3. The arm 26 makes contact at its free end with a further fixed contact 33 having an external terminal 34, to complete an alarm circuit that is connected between external terminals 32 and 34. When a plurality of these fuse devices are used together, it is convenient to arrange for them all to operate the same main alarm indicator (light or bell). It is also important to have an indication of which fuse has been ruptured, so that the attendant can go immediately to the ruptured fuse, as soon as he has been alerted by the general alarm. For this purpose a visual indication is provided by the slider 18 which is moved outwardly by the free end of the arm 25, as also shown in FIGURE 3. The projecting end 35 of this slider 18, which end may be differently coloured from the remainder of the casing and mount 11, in order to be readily visible, provides the desired visual indication. The slider 1S may be manufactured from coloured material to indicate the fuse rating. (Colour code).

A further feature of the device is a convenient means of testing. The attendant need merely push inwardly the slider 18, i.e., to the right from its FIGURE 2 position. He can do this, for example, by using some small pointed object such as the tip of a pencil pressed against the end 35. This pressure forces the slider 18 to the right until the free end of the spring arm 26 has been pushed into contact with the fixed contact 33 to temporarily energize the alarm circuit. In this way the fuse device can be readily tested without the need to expose the operator to `any electrically live connections. Both the spring arms and 26 are sufficiently flexible to permit their free ends to tlex during this operation, without causing any change to the location of the ledges 23 and 24 to which the two ends of the fuse wire 22 are connected. The location of the ledges 23, 24 is relatively firmly fixed by the fuse holder 17 against which they are held by tension in the wire 22.

The effectiveness of the particular structure of the fuse channel 19 is dependent largely on the existence of the central cavity 20. With the fuse wire 22 arranged as seen in FIGURE 4, the two end portions of the fuse wire are in direct physical and hence thermal conducting contact with the material of the fuse holder 17. On the other hand the central portion 21 of the fuse wire 22 which spans the cavity 20 is out of such contact. The fuse holder 17 acts as a heat sink, with the result that, when the wire is heated by Va current passed through it, the end portions of the wire that are in contact with the fuse holder do not rise to as high a temperature as the central portion, the wire having a substantially uniform rupturing capacity throughout its length. This result has two advantages.

Firstly, under high over-current conditions, the central portion of the wire reaches the rupture temperature before the end portions, with the result that the explosive action tends to take place in the centre. This location is both remote from the two ends and in a closely confined area. By making the channel 19 both narrow and shallow and the cavity 20 no larger than necessary to ensure lack of contact with the wire 22, the total volume of air sur rounding this part of the wire 22 can be kept low. As a consequence the violence of any explosive effect is similarly restricted. It will be appreciated that the accompanying drawings necessarily show the device larger than a typical size. Although the invention is in no way restricted to dimensions, it is pertinent to mention at this point that the present design enables the device to be conveniently small and yet able to rupture safely typical short circuit currents of the order of 1000 amps in a 300 volt circuit. By way of example, a typical length for the fuse wire 22 from ledge 23 to ledge 24 can be of the order of half `an inch.

Secondly, under normal usage with repeated heating and cooling of the wire, the higher temperatures are experienced in the central span 21. Thus, any tendency that there may be for the elevated temperatures to induce the metal of the wire to yield to the tension and Iadopt a permanent elongation, is substantially restricted to the central span 21. Since this span is only a fraction of the total length of the wire (preferably around 20%), the percentage elongation of the total length of the wire is only about one fifth of the percentage elongation suffered by the span 21. In other words, by thus exposing only a fraction of the wire to conditions conducive to permanent elongation, such elongation, in terms of a percentage of the total wire length, is kept low.

While 20% of the total wire length has been mentioned as a convenient length for the cavity 20, this fraction is not especially critical. The cavity length can be varied down to about 10% or up to around 40%. If the cavity is made too short, a suflicient temperature differential between the span 21 and the remainder of the wire is no longer achieved, since the wire span 21 will always lose some heat by conduction along the wire and hence to the material of the channel.v On the other hand, the longer the cavity is made, the greater will be the effect of permanent elongation of the span .on the total percenage elongation of the wire.

We claim:

1. An elec-tric fuse device comprising:

(a) a body of insulating material defining a confining channel,

(b) a length of fuse wire extending along said channel,

(c) meanssecured to respective ends of said wire for completing an electric circuit therethrough, including means for tensioning said wire, said tensioning means including a spring arm mounted at one end on said body and connected intermediate its ends to an end of said wire to hold said arm against its spring force and tension said wire,

(d) contact means located on said body for engagement by the other end of said arm upon its release by rupture of said wire, for completion of an external alarm circuit,

(e) and a movable member mounted on said body for movement by means of an exteriorly lapplied force to engage said other end of said arm to bend resiliently the portion of said arm lying between said other end and the connection of said arm to the wire, for forcing said other end against said Contact means for testing said alarm circuit.

2. An electric fuse device according to claim 1, wherein said tensioning means further includes:

(f) a second spring arm mounted at one end on said body and connected intermediate its ends to the other end of said wire to hold said second arm against its spring force and tension said wire,

(g) said movable member being mounted on said body for engagement by the other end of said second arm upon its release by rupture of said wire for movement of said member to a rupture indicating position.

3. An electric fuse device comprising:

(a) a body of insulating material defining a narrow, elongate` confining channel having a bottom surface comprising outwardly convex portions extending from each end of the channel towards an intermediate cavity forming a depression in the bottom surface of the channel,

(b) a length of fuse wire of substantially uniform rupturing capacity throughout its length and extending along said channel in contact with said convex surface portions and spanning said cavity,

(c) and means secured to respective ends of said wire closely abutting the ends of said channel for completing an electric circuit through the wire,

(d) said securing means including spring means for exerting a longitudinal tension on said wire to draw it tirmly into intimate heat conducting contact with said convex portions throughout the entire length of said wire between said securing means other than the portion thereof spanning said cavity.

4. An electric fuse device according to claim 3, wherein the length of said cavity in the direction along the fuse wire is within the range of approximately 10% to 40% of the total length of said wire.

5. An electric fuse device according to claim 3, wherein the length of said cavity in the direction along the fuse 5 6 Wire is approximately 20% of the total length of said (g) first contact means in electrical contact with said wire. first arm, for connection to an external alarm cir- 6. An electric fuse device according to claim 3, includcuit,

ing means connected to said spring means for detecting (h) second contact means located for engagement by movement thereof upon release of tension in the wire the other end of said first arm upon its release by consequent upon rupture of said wire. rupture of said wire, for completion of said external 7. An electric fuse device according to claim 6, includalarm circuit,

ing electric means for actuation by said detecting means (i) and a movable member mounted on said body for for providing an alarm indication of rupture of said wire. engagement by the other end of said second arm 8. An electric fuse device according to claim 6, includ- 10 upon its release by rupture of said wire, for moveing visual indicating means connected to said detecting ment of said member to a rupture indicating position. means for providing visual indication of rupture of said Wire, References Cited 9. An electric fuse device according to claim 3, where- UNITED STATES PATENTS in said spring means comprises:

wire, 2 T (f) a second spring arm mounted at one end on said 0 BERNARD A GILHEANY Primary Exammer body and connected intermediate its ends to the H. B. GILSON, Assistant Examiner other end of said wire to hold said second arm abutting the other end of the channel against its spring US- C1 X-R force and to tension the wire, 2&5 337-227 

